1. Field of the Invention
The present invention pertains to a method for producing a polymeric sol of a calcium phosphate compound, containing apatite with excellent bioactivity, according to a sol-gel synthesis, and a method for coating the same on a metal implant, in which the polymeric sol is coated on the metal implant and then heat-treated to form a dense-coated layer strongly bonded to the metal implant. More particularly, the present invention relates to a method for producing a polymeric sol of a calcium phosphate compound, in which the polymeric sol is transparent and homogenized and has excellent wettability because calcium and phosphate components are completely dissolved in the polymeric sol, and a method for coating a calcium phosphate ceramic, containing hydroxyapatite, on a metal implant frequently used in dental and orthopedic surgeries, in which the polymeric sol is coated on the metal implant and then heat-treated to form a dense coated layer strongly bonded to the metal implant.
2. Description of the Related Art
Grafting of a calcium phosphate ceramic, such as hydroxyapatite (HA, Ca10(PO4)6(OH)2), tricalcium phosphate (TCP, Ca3(PO4)2), tetracalcium phosphate (TTCP, Ca4P2O9), and calcium pyrophosphate (CPP, Ca2P2O7), into a bone tissue of a rabbit has been conducted by Kitsugi et al., as disclosed in “Biomaterials” 16, 1101-1107 (1995). With respect to this, they observed an interface between the calcium phosphate ceramic and the bone tissue using a transmission electron microscope (TEM), resulting in the finding that the calcium phosphate ceramic was chemically combined with the bone tissue.
The above examples of the calcium phosphate ceramic are all chemically combined with the bone tissue, but different from each other in terms of its dissolution speed in humans. Of them, hydroxyapatite is known as the most stable calcium phosphate ceramic having the most similar chemical properties to inorganic materials constituting the bone tissue of humans in the case of grafting hydroxyapatite into humans. Accordingly, a lot effort has been made in developing a hydroxyapatite ceramic clinically used as an artificial bone material. However, the hydroxyapatite ceramic has relatively high brittleness considered as a disadvantage of a traditional ceramic material even though it has excellent biocompatibility. Hence, an implant made of a metal material, such as stainless steel, cobalt-chromium alloy, and titanium alloy, is frequently used as a structure which must endure a repeating load.
To combine excellent physical strength of a metal material with the high biocompatibility, various studies have been made to coat hydroxyapatite on a surface of the metal implant. The most traditional process of coating hydroxyapatite on the surface of the metal implant is a plasma spray process. According to the plasma spray process, hydroxyapatite powder is moved through a plasma region at 20000 to 30000° C. using a carrier gas to be instantaneously fused, and then bonded to a target substrate. The plasma spray process is more advantageous than the other coating processes, such as a sputtering process and a chemical vapor deposition process, in terms of bond strength. In other words, when the hydroxyapatite powder is bonded to the target substrate according to the plasma spray process, the bond strength of the hydroxyapatite powder to the target substrate is relatively high.
However, when hydroxyapatite particles pass through the very hot plasma region, chemical structures of hydroxyapatite particles are destroyed to release decomposed components, such as tricalcium phosphate, tetracalcium phosphate, calcium oxide, and amorphous calcium pyrophosphate, to enable a coated layer on the target substrate to be absorbed into the body when the coated target substrate is grafted into the body, leading to the dissociation of the coated layer from the target substrate or the reduction of strength of the coated layer (refer to Filiaggi et al. J. Biomed. Mater. Res., 25:1211-1229, 1991).
A sintering temperature of the hydroxyapatite ceramic is 1100° C. or higher, which acts as an obstacle in a developing an improved coating process to avoid the above disadvantages. The metal implant mostly consisting of stainless steel 316 L and titanium ally (Ti-6Al-4V) is largely reduced in terms of the physical strength and is severely oxidized at 1100° C. or higher, and thus, the hydroxyapatite coated layer must be densely sintered at 1000° C. or lower.
Recently, a lot of effort has been made to synthesize the nano-sized hydroxyapatite particles (1 nm=10−9 m) to reduce the sintering temperature of the hydroxyapatite ceramic. For example, reference may be primarily made to a sol-gel synthesis, in which metal organics are hydrolyzed to form first particles of three to four nm, and the first particles are subjected to a polycondensation reaction to form gel meshes. When a solvent is removed from the gel meshes, the gel meshes are dried and shrunken, and the shrunken gel meshes are heat-treated to produce final ceramic particles. The sol-gel synthesis is advantageous in that because the ceramic particles are chemically uniform and largely reduced in terms of a size, the ceramic particles have excellent reactivity. However, the sol-gel synthesis is applied to only a special field, such as a coating process, because an amount of the ceramic particles produced according to the sol-gel synthesis is relatively small.
When the hydroxyapatite ceramic produced according to the sol-gel synthesis is coated on an objective body, the hydroxyapatite ceramic is crystallized at relatively low temperatures and its sintering temperature is largely reduced, providing a novel apatite coating process. In this regard, the preferable selection of calcium salts, phosphates, and the solvent is the most important factor in the sol-gel synthesis of the calcium phosphate ceramic. Additionally, in the sol-gel synthesis, it is necessary to ensure a desirable aging condition in which calcium and phosphates are uniformly mixed with each other and sufficiently come into contact with each other.
U.S. Pat. No. 5,766,669 discloses a method of coating an apatite sol on a metal substrate, including mixing calcium nitrate (Ca(NO3)2) with an ammonium phosphate solution to produce the apatite sol, coating the apatite sol on the metal substrate, and heating the resulting metal substrate at 950 to 1000° C. Furthermore, U.S. Pat. No. 6,569,489 discloses a method of coating apatite on a substrate, including dipping the substrate in an aqueous solution, containing calcium, phosphate, and carbonate ions maintained at 100° C. or lower within a pH range of 6.0 to 7.5 for a sufficiently long time, to chemically react the aqueous solution with the substrate to form a crystalline apatite coated layer on the substrate.
Meanwhile, D. M. Liu et al. attempted the coating of an apatite sol on a pure titanium substrate, in which a solution of calcium nitrate (Ca(NO3)2) in anhydrous ethanol is mixed with another solution, obtained by adding triethyl phosphite and water into anhydrous ethanol and hydrolyzing the resulting mixture, to produce the apatite sol, as indicated by “Biomaterials” 22 1721-1730 (2001). Additionally, K. A. Gross. et al. suggested a technology of producing of an apatite coated layer, including mixing a first solution of calcium ethoxide in ethanol and ethane diol with a second solution of triethyl phosphite in ethanol and ethane diol to produce an apatite sol, coating the apatite sol on a titanium substrate, and heating the resulting substrate at 800° C., in “J. Mater. Sci. : Mater. In Med. ” 9 839-843, (1998).
However, it is undesirable to coat the substrate with the use of an apatite aqueous solution or to use calcium nitrate as the calcium salts because the coated layer with an nonuniform thickness is formed on the substrate due to poor wettability of the apatite sol to the substrate when the apatite aqueous solution is coated on the substrate.
Like in the case of using water as the solvent, in the case of using the mixed solvent of ethanol and ethane diol, the wettability of the apatite sol to the substrate is reduced, preventing the uniform and dense coated layer form being formed on the substrate. Generally, a sol with excellent wettability is very useful to form the coated layer with the uniform thickness because it is uniformly dispersed on a surface of the substrate. Therefore, one of the most important factors in a sol-gel coating method is the wettability of the sol, produced according to the sol-gel synthesis, to the substrate.
A conventional, commercial coating method using the apatite sol has a disadvantage in that it is impossible to form the dense coated layer with the uniform thickness because of the use of the apatite sol with poor wettability. Hence, the coating method using the apatite sol has not been widely utilized even though the coating method is advantageous in that the method is simply conducted and it is not necessary to use costly coating devices.